While the 3GPP LTE standard (e.g., 3GPP LTE release 8 or 9) supports uplink transmission from a user equipment (UE) through a single transmit antenna, the 3GPP LTE-A standard discusses support of uplink transmission (uplink MIMO transmission) from a UE through a plurality of transmit antennas in order to increase uplink transmission throughput.
Multi-antenna transmission is also called Multiple Input Multiple Output (MIMO). MIMO can increase the efficiency of data transmission and reception using multiple transmit antennas and multiple receive antennas. MIMO schemes include spatial multiplexing, transmit diversity, beamforming, etc. A MIMO channel matrix formed according to the number of receive antennas and the number of transmit antennas can be decomposed of a plurality of independent channels and each independent channel is called a layer or stream. The number of layers or streams or a spatial multiplexing rate is called a rank. In UL MIMO transmission, transmission can be performed through a plurality of layers and the same data or different data (i.e., codeword or transport block) can be transmitted through each layer.
For an independent codeword transmitted on uplink, a receiving side (e.g., eNB) can check whether the codeword has an error through cyclic redundancy check (CRC) and generate acknowledgement (ACK) information. A reception state may be represented as ‘ACK’ when no error is generated and may be represented as ‘NACK’ when an error is generated. The receiving side transmits the generated ACK information to a transmitting side (e.g., UE) such that the transmitting side can perform hybrid automatic repeat request (HARQ) transmission. According to HARQ transmission, the transmitting side retransmits data previously transmitted thereto when receiving a NACK signal from the receiving side and the receiving side combines previously received data and the retransmitted data to improve retransmission performance.